There is no critique in the class material of the concept of “velocity” of money. The Austrian analysis of the purchasing power of money or price inflation is in the lecture on the “Money Market” in the course on Austrian Economics.
Mises has a critique of the quantity theory of money in his book, The Theory of Money and Credit:
The basic point is that nothing more can be learned about the price of a good by using the concept of velocity. The price of cars is determined by demand and supply. We don’t need to know anything about the “velocity” of cars to understand how the price is determined. The price of money or its purchasing power is also determined by demand and supply. Velocity of money adds no additional insight to the analysis.